Generally, as shown in FIG. 1, a video signal processor for next generation digital video apparatuses, such as high-resolution digital televisions and high-resolution digital video cassette recorders, converts an analog composite video signal input from an input source (not shown in FIG. 1) into a digital signal in an A/D converter 10. The digital video signal is digitally processed by first and second signal processors 20 and 30 and the digital video signal is output as luminance and color difference signals Y, R-Y and B-Y to a D/A converter 40. D/A converter 40 then converts the luminance and color difference signals into R, G and B analog color signals.
As shown in FIG. 2A, the normal transient rise time (hereinafter referred to as rise time) of luminance signal Y output from first signal processor 20 is about 150 nanoseconds (nsec) while, as shown in FIG. 2B, the rise time of color difference signals R-Y and B-Y output from the second signal processor 30 is about 800 to 1200 nsec.
The frequency bandwidth of a color difference signal is much smaller than that of a luminance signal. In a standard television system, the bandwidth of the color signal is smaller than that of the luminance signal by about a factor of five.
In other words, since the transmission frequency bandwidth of a color subcarrier is relatively narrow and the transient time of the color signal is about 5 times longer, as compared with that of the luminance signal, if the color is abruptly changed, the color edge becomes unclear.
According to the above apparatus, since the rise times of the color difference and luminance signals do not match and the rise time of the color difference signal is far longer than that of the luminance signal, when the digital luminance and color difference signals are converted into analog video signals to be displayed on a display, the sharpness of a picture's edge is reduced, resulting in reduced picture quality.
A circuit for steepening the transient slope of the color signal is disclosed in Japanese Laid-Open Patent Publication SHO 61-244186, which consists of a slope detector, a memory, a switch and a timing controller. Time threshold value and amplitude threshold value signals are supplied to the slope detector so as to steepen the slope of the color signal for operating conditions which are above the amplitude threshold value and below the time threshold value. However, the transient time of the color signal does not always match that of the luminance signal in the above mentioned apparatus.